decreased B-1 B cell number absent B-1 B cells MP:0008202 MPheno.ontology absence of the subset of B cells found predominantly in the peritoneum, pleural cavities, and spleen, and are enriched for self-reactivity; B-1 B cells are thought to be the primary source of natural IgM immunoglobulin, that is, IgM produced in large quantities without prior antigenic stimulation and generally reactive against various microorganisms, as well as the source of T-independent IgA immunoglobulin in the mucosal areas absent mature B cells